Observational aspects of magnetic activity in stars are discussed with emphasis on their implications for dynamo models. The dependence of chromospheric and coronal emission on rotation in convective stars is a strong, albeit indirect evidence for the occurrence of some kind of dynamo action in all late-type stars. The available data, including those on stellar activity cycles, are as yet insufficient to constrain dynamo models in an effective way. The stellar data, anyway, seem to suggest a much more complex picture of the dynamo process than the one indicated by solar observations.